Curtis, a first-term representative from Butte, now faces the challenge of making her case for Montana voters to choose her over well-known and well-funded Republican Rep. Steve Daines with less than three months until the Nov. 4 election.

"If we win here in Montana, outspent and outgunned in a race where we were left for dead, it will send a message to Washington, D.C., that we want change," she said in a speech before Saturday's vote.

Republicans need a net gain of six seats in November to take Senate control, and Montana is a prime target to pick up a seat that's been in Democratic hands for more than a century.

The Senate race was seen as a tough one for Democrats to win even with the incumbent Walsh in the running. Now Daines is expected to have a bigger advantage going against a newcomer who doesn't have his name recognition or $1.7 million campaign bank account.

"We need to keep this seat, period," said Judith Forseth, a delegate from southwestern Montana's Park County. "I'm not ready to concede."

Curtis, 34, is a high school math teacher. She emerged as the front-runner last week after she received the endorsement of Montana's largest unions, and high-profile party leaders said they weren't interested in running.

Daines' campaign released a statement after the selection that said voters will have a clear choice between the candidates.

"I respect Montanans' judgment to decide the path that is best for our state, and remain focused on fighting for positive solutions that protect Montanans from Washington overreach, grow our economy, and get our country back on track," he said.

Curtis drew 82 votes versus the 46 cast for Wilsall rancher Dirk Adams, a political newcomer who lost the Democratic primary to Walsh on June 3.

Montana Gov. Steve Bullock appointed Walsh, who was his lieutenant governor, in February when Max Baucus resigned from the Senate after 35 years to become ambassador to China.